The transfer of torque between a first rotating device and a second rotating device that rotates at a different speed from that of the first rotating device, or between a rotating device and a stationary device is a common requirement in mechanical systems. The ability to transfer energy without physical contact between machine elements is necessary to avoid wear and tear of the machine elements caused due to tribological interactions between the machine elements. An example of an electrical machine that possesses the ability to transfer energy without physical contact between machine elements is an eddy current slip coupling system with an infinitely variable torque transmission. Typical eddy current slip coupling systems comprise relatively moving or slipping driving members and driven members composed of magnetic materials. One of the members may contain a thin plate of copper or a similarly conductive non-magnetic material facing. In these eddy current slip coupling systems, a field winding is used to generate a flux field that interlinks the driving member and the driven member. One of the members generates a concentrated flux, while the other member that receives the concentrated flux is an armature. The relative motion or slip between the driving member and the driven member produces movements of flux field concentrations in the armature. The slip and field excitation produces eddy currents in the armature, which in turn produces mechanically reactive flux, through which torque is transmitted between the driving member and the driven member.
Eddy current coupling systems have been available in different configurations for several years. In conventional systems, eddy currents are generated in either a radial design or an axial design. In such conventional systems, the armature receives the concentrated flux through an air gap in either a radial direction or an axial direction, which leads to a variation in torque transfer as the rotational speed changes. The retarder system manufactured by Valeo Corporation/Telma Retarder Inc., is an example of an axial system. The coupling systems manufactured by Dynamatic® of Omnidrive Holdings, LLC, utilize a radial eddy current system. Therefore, there is a need for an eddy current coupling assembly that generates eddy currents in both radial and axial directions for achieving uniform torque transfer characteristics over a range of rotational speeds.
The use of electrically conductive non-magnetic materials in eddy current coupling systems is currently available. However, although these materials increase the strength of the eddy currents generated, these materials also increase the magnetic reluctance of the magnetic circuit, which in turn requires more power to achieve the same flux levels. Moreover, higher differential speeds increase the frequency of the eddy currents that cause skin effects to limit the penetration of the eddy currents into the conductive material, resulting in a drop in torque transfer efficiency. As used herein, the term “skin effect” refers to a tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with increasing depths within the conductor. The electric current flows mainly at the skin of the conductor, between the outer surface and a level called skin depth. Due to the manufacturing difficulties associated with applying layers of materials in a radial design, the layers of these materials are typically utilized in the axial designs. Therefore, there is a need for an eddy current coupling assembly that transfers high levels of torque across an entire slip speed range while keeping the coupling as compact as possible. A ratio between a rotation rate of a magnetic field as seen by a rotating member and a rotation rate of a magnetic field as seen by the other rotating member is referred to as the slip speed. Moreover, there is a need for reducing the variation of torque transfer as a function of slip speed, making torque transfer more uniform for a given field excitation throughout a range of slip speeds. Furthermore, there is a need for enhancing the torque transfer of the eddy current coupling assembly in different slip speed regions.
Hence, there is a long felt but unresolved need for an electrical slip coupling assembly that generates eddy currents in both radial and axial directions and that allows manipulation of torque transfer characteristics as a function of slip speed across a range of slip speeds for a given field excitation current using different configurations of a field member and an armature member of the electrical slip coupling assembly.